Evolution gets spin in study

Scientists see more recent chimp-human link in DNA.

Scientists see more recent chimp-human link in DNA.

May 18, 2006|DAVID BROWN The Washington Post

When the ancestors of human beings and the ancestors of chimpanzees parted ways 6.3 million years ago, it was probably a very long goodbye. Some of their descendants may even have gone back for a final tryst. That's the conclusion a group of scientists has reached, using a comparison of the genes of human beings and their closest animal relatives to sketch a picture of human origins far more detailed than what fossil bones have revealed. According to the new theory, chimps and humans shared a common, apelike ancestor much more recently than previously thought. Furthermore, when the two emerging species split from each other, it wasn't a clean break. Some members of the two groups seem to have interbred about 1.2 million years after they first diverged -- or about 5.4 million years ago -- before finally going their separate ways for good. If this theory proves correct, it means modern people are descended from something akin to chimp/human hybrids. That's a new idea, and it challenges the prevailing view that hybrids tend to die out. It also strongly suggests that some of the oldest bones of "proto-humans" -- including the 7-million-year-old Toumai skull unearthed in Chad in 2001 -- may have belonged to a line of nonhybrids that died out, and were not human ancestors at all. This narrative, by a team of geneticists and biostatisticians from the Broad Institute of Harvard and Massachusetts Institute of Technology, not only casts new light on the origin of humans, it raises questions about how all new species arise. "This is contributing to the idea that species are kind of fuzzy. They become real over time, but it takes millions of years," said James Mallet, a geneticist at University College in London not involved in the new research. "We probably had a bit of a messy origin." The research is the latest fruit of the Human Genome Initiative, the successful effort to transcribe and read out the entire genetic message of human chromosomes, which was completed in 2003. The evidence of ancestral chimp and human interbreeding emerged from comparing parts of their genomes to each other and to those of gorillas, orangutans and macaques. The scientists now want to know whether similar "hybridization events" happened between other emerging species. The separation into two species "left a footprint on our genome that we can go back and read," said Eric Lander of MIT. "We were never able to look at things like this before. What we need to do now is to collect more data and look for other smoking guns."